1. Field of the Invention
The present invention relates to a laser irradiation method for irradiating laser beams and to a laser irradiation apparatus (an apparatus including a laser and an optical system that leads laser beams produced from the laser to an irradiation object) for implementing the irradiation method. Further, the present invention relates to a manufacturing method for a semiconductor device that is manufactured according to steps including irradiation of a laser beam. The semiconductor device mentioned above refers to an electrooptical device such as a liquid crystal display device or a light-emitting device and to an electronic device including the electrooptical device as a component.
2. Description of the Related Art
Techniques having widely been researched in recent years include those for producing a crystalline semiconductor film in such a manner that a semiconductor film formed on an insulating substrate formed of, for example, a glass substrate is annealed with laser beam light and is crystallized to improve crystallinity. Throughout the present specification, the crystalline semiconductor film refers either to a semiconductor film including a crystallized area or to a semiconductor film crystallized overall.
Compared with a synthetic quartz glass substrate, a glass substrate is advantageous in that a large area substrate can easily be produced at low costs. The laser is frequently used in crystallization for the glass substrate, because the glass substrate has a low melting point. The laser is advantageous in that it imparts a high energy to a semiconductor film without greatly increasing the temperature. Further, a significantly high throughput can be obtained in comparison to the case of a heat treatment performed with an electrothermal oven.
A crystalline semiconductor film formed with laser beam irradiation exhibits a high mobility. Thus, the semiconductor film is used for, for example, an active-matrix liquid crystal display device. The active-matrix liquid crystal display device is manufactured such that thin film transistors (TFTs) are formed using crystalline semiconductor films on a single glass substrate so that the TFTs are used for either pixel portions or pixel portions with a driver circuit.
Laser annealing is performed in a method in which a high-power pulse laser beam generated by an excimer laser or the like is processed by an optical system to be shaped as, for example, a several centimeter square spot, a 10 centimeter or longer line, or a rectangle, on an irradiation surface. Concurrently, the laser beam is scanned (alternatively, an irradiation position of the laser beam is moved relative to the irradiation surface). Since this method exhibits high productivity and provides industrial advantages, the method is preferentially employed.
In particular, according to the method of using the spot-like laser beam, scanning needs to be performed backward-forward and rightward-leftward on an irradiation surface. However, according to the method of using the linear laser beam, the laser beam can be irradiated to the entirety of the irradiation subject surface only by performing scanning in a direction perpendicular to the linear direction of the linear beam. The scanning is thus performed in the direction perpendicular to the linear direction for the reason that the direction is the most effective in terms of productivity. As a current mainstream technology, laser annealing is performed using the method in which a laser beam emitted from an excimer laser of a pulse-oscillation type is processed using an appropriate optical system, and a beam thereby shaped linear on an irradiation surface is used for irradiation.
As an example, FIGS. 6A and 6B show a configuration of an optical system disclosed in JP 2001-244213 A. The optical system is used to process a laser beam on an irradiation surface and to thereby shape the beam linear. First, the configuration will be described with reference to a side-view in FIGS. 6A and 6B. A laser beam passed through a laser oscillator 101 travels straightforward with a divergence angle. Subsequently, the laser beam is processed into collimated beams through cylindrical lenses 104 and 105, and split beams then converge on an irradiation surface 107 via a mirror 106. Since the optical system shown in FIGS. 6A and 6B are easily influenced by variations in the divergence angle, the system needs to be controlled. Thus, the configuration preferably uses an optical system that produces collimated beams that are not influenced by the divergence angle. Although a completely collimated beam cannot be produced, divergence of the beam can be minimized. An optical system of the type described above is called a “beam collimator”. In the configuration shown in FIGS. 6A and 6B, the cylindrical lenses 104 and 105 serve as collimators. The mirror 106 is shaped to have a plurality of parabolic mirrors each having a different curvature (mirrors each having only a unidirectional curvature), on which the beams converge at the focal points and are then led to reach the irradiation surface 107. In the example, four parabolic mirrors are provided. Since the curvatures of parabolic surfaces are different from one another, the focal points are also different from one another. According to parabolic mirrors 106a to 106d, energies of linear beams are homogenized in the beam cross direction, and the beam lengths thereof are determined.
Next, the configuration will be described with reference to a top view in FIG. 6B. A laser beam passed through the laser oscillator 101 is split through a cylindrical array lens 102 in a direction perpendicular to the traveling direction of the laser beam light. In the present specification, the aforementioned direction is referred to as a “transverse direction”. With a mirror inserted in the course of the optical system, the transverse direction is variable to a direction of light bent by the mirror. In this configuration, the laser beam is split into four laser beams. The split laser beams converge through a cylindrical lens 103 into a single laser beam on the irradiation surface 107.
Generally, as a lens for transmitting laser beams is repeatedly used, the lens gradually deteriorates and becomes unusable. However, dissimilar to such a lens, a mirror does not transmit laser beams but reflects the laser beams impinged against its surface. That is, only the surface of the mirror is deteriorated. Thus, even in a case where the mirror is used for a long time to an extent of causing the surface thereof to deteriorate, the mirror can be rectified to be usable by recoating the deteriorated surface. From this viewpoint, the mirror is economically advantageous.
However, an energy density distribution of a laser beam formed on the irradiation surface 107 according to the optical system shown in FIGS. 6A and 6B are as shown in FIG. 7. Such a distribution is attributed to continual variations in the energy density. The variations are caused due to lens aberrations in the individual mirrors and differences in optical lengths to the irradiation surface. Since inhomogeneous energy density distributions of the aforementioned type with individual mirrors are synthesized on the irradiation surface 107, inhomogeneity is increased significant.
The energy density distribution of the beam formed on the irradiation surface 107 is preferably homogeneous to perform homogeneous laser annealing for an irradiation object. For example, when the energy density is homogeneous in processing a semiconductor film used as an irradiation object, the semiconductor film can be annealed homogeneous, thereby having homogeneous physical property. Consequently, a TFT manufactured using the semiconductor film thus annealed reduces the inhomogeneity in electrical characteristics. Consequently, improvement can be attained in operational characteristics and reliability of a semiconductor device produced using TFTs manufactured in the manner described above.